Acceleration sensor

ABSTRACT

A low-cost breakable inertial threshold sensor using mainly micro-machining silicon technology constructed on a silicon-wafer or on some other brittle material according to the MEMS process. The sensor comprises a first body portion, a second body portion, and detecting means for giving an indication if the second body portion has damaged the detecting means. The status of the sensor can be read in various ways. In one embodiment the status is remotely readable.

FIELD OF THE INVENTION

The present invention relates generally to micro-electromechanicalsystems (MEMS) and in particular to the structure and operation of amicro-mechanical acceleration sensor.

BACKGROUND OF THE INVENTION

There is a growing trend toward smaller and smaller components forelectrical applications. Thus micro-electromechanical (MEM) systems andmicrosystems technology (MST) have made rapid progress in recent years.MEMS/MST technology has the advantages of reliability, small size, lowweight and low cost. MEMS is probably best known for its sensor andactuator applications.

The substrate material most used in the production of micro-electroniccircuitry is silicon (Si). Among other suitable materials used aresilicon dioxide (SiO₂), silicon nitride (SiN), polycrystalline silicon,and quartz.

FIG. 1 a is an explosion view of a prior-art acceleration sensor basedon the inertia of a silicon proof mass. The acceleration sensor has amultilayer structure comprising silicon layers 101, 111, 121 and glassinsulator layers 102 and 122, of which the former is located betweensilicon layers 101 and 111 and the latter between silicon layers 111 and121. The structure further comprises two stationary capacitor plates, ofwhich the first plate (not shown in the figure) is between a glassinsulator layer 102 and a silicon layer 111 and the second plate 124 isbetween the silicon layer 111 and a glass insulator layer 122. Siliconproof mass 104 is fastened to the frame 111 via two elastic siliconsprings 105. Between the proof mass and each glass insulator there istypically a one-micrometer space.

When the acceleration sensor is subjected to acceleration, elasticsprings 105 balance the inertia of the proof mass by bending. Therelatively small displacement of the inertial mass is measured bycomparing the capacitance formed by the second plate 124 and the proofmass with the capacitance formed by the first plate and the proof mass.The electrical connections needed for said comparison are formed bymetal films 103, 113, and 123 arranged on corresponding outer surfacesof each of the silicon layers.

An accelerometer such as the one depicted in FIG. 1 a can easily beconstructed to measure the desired acceleration range. However,drawbacks are that it is expensive to manufacture and requirescomplicated measurement electronics needing a power supply.

FIG. 1 b depicts a top view of a prior art latching accelerometer thatmechanically records shocks without needing a power supply. Such alatching accelerometer can be used as a peak-reading shock recordertypically covering the range from 60 g to 3500 g (g is acceleration ofgravity).

The main parts of the accelerometer are a pedestal 151, a flexiblecantilever 153 with one end fixed to the pedestal, an inertial mass 152in the middle of the cantilever, and a number of notches 154-158 formingan arc. Movement of the cantilever is prevented by dose glass and/orsilicon surfaces (159 and 160) on either side of the cantilever in theplane parallel to the paper.

When the accelerometer is subjected to acceleration the inertial massdeflects the cantilever tip 158. Depending on the amount of accelerationthe tip moves from one notch to another. For example, if the originalposition of the tip is between notches 155 and 156, it may move tobetween notches 154 and 155 or alternatively to between notches 156 and157.

The spring force of the elastic cantilever (dimensions are typically:length 1 mm and thickness 5 μm) is not strong enough to return the tipto the original position. Additional stops 161 and 162 can be arrangedthat allow incremental thresholds to be recorded.

Although this accelerometer is adequate for many purposes, typically asindicators of rough handling in shipping operations according to aneditorial article in Electronic Design Magazine of Jun. 23, 1997, pp.28-31, the mechanically latching accelerometer has several drawbacks.Due to friction the acceleration threshold is hard to control precisely,and variation between individual units is high. Additionally, theinertial mass indicates acceleration in one plane only.

Most of the MEMS accelerometers developed are based on an inertial proofmass, which acts on a spring or springs, and the deflection from theidle position is measured. For example, a capacitive circuit element canbe made to change capacitance depending on this deflection. Automobileair-bag accelerometers typically use this measurement method and havebeen developed into reliable mass-produced low-cost devices.

Micromechanical lateral field emitters arranged on bending cantileversare used in some acceleration sensors. In such sensors the strength ofcurrent is based on the bending of the cantilever, which deflectslateral field emitters from opposing each other. The acceleration sensorneeds supporting electronics in both said prior-art cases. However, thisincreases the production costs, which is not acceptable in many cases.

Prior-art acceleration sensors are generally discrete devices where thesensor and the measurement electronics are implemented on separatechips. Lately some surface micromachined acceleration sensors have beendevised, where the sensor and the measurement electronics areimplemented on a single semiconductor chip. Such sensors are generallypackaged in single chip modules (SCM) or multichip modules (MCM)featuring both the sensor and the measurement electronics in the samepackage.

One drawback is that at the moment there is no such sensor commerciallyavailable, suitable for mass-produced handheld terminals, such aselectronic books, with the capability to register or warn when theterminal has suffered an acceleration shock. Additionally, no method isprovided for remote reading in the prior-art sensors, or for timeregistering shock events.

Normal practice is that companies provide a warranty for products suchas electronic equipments. If any faults or defects are found during awarranty period, the customer has the right to claim either repair orreplacement of the faulty equipment free of charge. However, there is nomethod to find out whether the customer has handled the electronicdevice too roughly or whether the device was already damaged whenreceived. Usually the product itself does not in any way inform eitherthe user or the repairman of mishandling if no visible physical damagecan be found. Unnecessary warranty repairs in consequence of mishandlingare common today due to the fact that the cause of breakage or damage isuntraceable. From the manufacturers' and dealers' point of view this isfrustrating and often very uneconomical. Mishandling could be minimizedif the product in one way or another warned the user of rough usagewhich could damage the product.

Furthermore, illegitimate warranty claims could be avoided if theproduct itself could indicate abusive handling. Mishandling of lentequipment could also be avoided if the borrower knows that anymishandling can be ascertained when the equipment is returned.

SUMMARY OF THE INVENTION

An objective is to implement a low-cost breakable inertial thresholdsensor using mainly micro-machining silicon technology. Other objectivesare that the sensor is suitable for mass production, small in size andsurface mountable, with the possibility to register accelerationthreshold levels. The sensor can be used to check if a device such as amobile terminal has suffered a drop or any other acceleration shock. Atthe moment there is no suitable sensor available for this kind ofpurpose.

A sensor is constructed on a silicon wafer or on some other brittlematerial according to the MEMS process. The sensor is constructed from afirst body portion, a second body portion, an interconnecting elementmaking the first body integral with the second body, and detecting meansfor giving an indication if the second body portion damages thedetection means.

In one embodiment of the invention the sensor comprises at least oneinertial mass with at least one tiny breakable cantilever such as abracket, beam, or bar one end of which is connected to the inertial massand the other end to a supporting frame. The size of the cantilever isquite small in comparison with the inertial mass. When the sensor isaccelerated the inertial mass causes stress on the cantilever resultingin its rupture at a certain stress level.

Information about the breaking sensor can be measured by means of changeof electrical impedance, for example. The sensor may be covered at leastpartly with some conductive material that breaks along with thecantilever or the movement of the inertial mass may break the conductivepath. Other means to detect a broken cantilever is to measure theresponse of the inertial mass to actuation by mechanical orelectro-magnetical means. Since the broken device response is differentfrom the unbroken one, capacitive reading can be applied. The preferredmethod is however to measure the conductance.

The breakage of the conductive path, with the time of occurrence and thedirection of the acceleration, can be recorded and read either activelyor passively depending on the solution used. Several sensors respondingto a different force may be implemented in the same product. The statusof the sensor is readable either directly or from a memory. There arevarious alternatives for reading the status of the sensor such asself-test reading, online reading or remote reading. Even if the amountof interconnections within the sensor group are minimized, the status ofthe sensors of a sensor group can simultaneously be read when it isbeing ascertained whether one or more sensors are broken.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described more closely with reference to theaccompanying drawings, in which

FIG. 1 a illustrates an example of a prior art acceleration sensor,

FIG. 1 b illustrates an example of a mechanically latching prior artaccelerometer,

FIG. 2 illustrates an example of the basic structure of the inertialmass of the sensor,

FIG. 3 a-c illustrates some examples of the basic structure of theinertial mass of the sensor,

FIG. 4 a is a typical strain-stress graph for polycrystalline materialsand metals,

FIG. 4 b is a typical strain-stress graph for single crystal materialssuch as single crystal silicon,

FIG. 5 illustrates one example of a structure including more than oneinertial mass etched on the same chip,

FIG. 6 illustrates a process flow to produce the micromechanicalinertial sensor,

FIG. 7 a-b illustrates a cross section, top, and front-side views of theproduction sample according to the process flow,

FIG. 8 a-c shows one example of a breakable acceleration sensor,

FIG. 9 illustrates the reading of sensor loops,

FIG. 10 exemplifies remote sensing using RF resonance circuits,

FIG. 11 a-b illustrates an example of an acceleration sensor processedusing both bulk-mechanic and surface micro-mechanic technologies, and

FIG. 12 a-b illustrates an acceleration sensor system on a substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A small breakable inertial threshold sensor according to the inventionmay be used for checking if a product or a device such as a mobileterminal has suffered a drop or other acceleration shock. The operationof the sensor is based on the simple fact that when the sensor isaccelerated the sensor inertial mass which is connected to a supportframe by at least one tiny beam, will oppose the movement. When theopposing inertial force is great enough, the beam will break.

In the following the small breakable inertial threshold sensor producedby micromachining technology is considered in more detail by way ofexamples in FIG. 2-11. It is to be noted that the relative dimensions ofthe components shown in the figures may vary in reality. The inertialthreshold sensor is hereinafter called an acceleration sensor or shortlya sensor.

First the inertial mass with at least one cantilever beam apart from asupporting frame is considered in detail.

FIG. 2 illustrates an example of the basic structure of an inertial massof the sensor. The structure is cubical comprising two tiny cantileverbeams 201 symmetrically at the opposite corners on the same edge of thecubical mass. The inertial mass is a few hundred micrometers thick andmade of commercially available Silicon On Insulator (SOI), which is acomposite structure consisting of two silicon layers 200 with a thininsulating layer (e.g. silicon oxide SiO₂) 203 between them. In thisexample, the top silicon layer is covered with a conductive material 204which also covers the top layer of the cantilever beams as shown in FIG.2. Examples of other insulators include silicon nitride and aninsulating form of silicon. The conductive layer can be polycrystallinesilicon, metal, or any other suitable breakable conductive material.

One end of the cantilever beam is connected to the inertial mass and theother end to a frame (not shown in FIG. 2). The cantilever beam is quitesmall in comparison to the inertial mass. Thus, should a sufficientexternal accelerating force be applied to the sensor, the accelerationinduces movement of the inertial mass causing stress on the cantileverbeams to rupture at least one of them and simultaneously to break of theconductive path.

Information about the breaking sensor fuse can be measured by means of achange of path impedance in the conductive part, for example. Theinertial mass of the sensor determines its response to an acceleratingforce. For example, the structure in FIG. 2 is the most sensitive forforces acting on it from the direction parallel to the z-axis and lesssensitive to forces acting on it from the direction parallel to they-axis. Strictly speaking the z-component of the torque T is greaterthan the x- and y-components.

In general the rigid body has three degrees of freedom in relation totranslation, one in each direction parallel to the axis of thecoordinates. Similarly it has three degrees of freedom in relation torotation, one around each axis of the coordinates. In order for the bodyto be in equilibrium, the sum of all forces acting on it must be zero,as well as the sum of all torques.

The sensitivity of the sensor can be adjusted to the required level bychanging the cantilever and/or mass dimensions. The sensitivity of thesensor depends also on the location of the cantilever beams, as well astheir shape. Further, the number of cantilever beams has an effect onthe sensitivity. The rupture point of the cantilever beam can becalculated and adjusted by altering the parameters: length l, width b,and thickness h of the cantilever beam and dimensions of the inertialmass.

FIG. 3 a-3 c illustrates some examples of the basic structures of theinertial mass of the sensor. Though the inertial mass with cantileverbeams is shown in the figures apart from the frame, it is to be notedthat they can all be etched on a SOI wafer (this will be studied laterin greater detail) so that the interface between the cantilever beamsand the frame is unbroken.

The sensor according to the invention comprises an inertial mass, atleast one breakable cantilever beam, and a frame supporting the inertialmass from one end of said cantilever beam. The cantilever beam islocated preferably in a corner on the edge of the inertial mass as shownin FIG. 3 a, and 3 b. However, depending on requirements the cantileverbeam(s) may be located anywhere in the inertial mass (FIG. 3 c),symmetrically or asymmetrically. Of course, the required accelerationthreshold level determines the size, the shape, and the weight of theinertial mass used, as well as the number and location of cantileverbeams 301. The acceleration threshold level can be from 500 g to 10000g, where g is the acceleration of gravity. For example, if a body isreleased from rest at a height of one meter and its stopping distance isone millimeter, the deceleration applied to the body is about 1000 g. Itis to be noted that the threshold level limits mentioned above are onlyguidelines.

Assume that in FIG. 3 a-3 c the inertial mass of the sensor is made ofcommercially available Silicon On Insulator (SOI), which is a compositestructure consisting of two silicon layers 300 and an insulating layer303. The insulating layer, however, is not necessarily required in themanufacturing process, i.e. when the inertial mass is etched theinsulating layer merely facilitates the processing. At least part of theouter surface of the sensor is covered with some breakable conductivematerial (not shown in FIG. 3). As described above the cantilever beamwill break if an acceleration being applied to the sensor is greatenough to tilt the inertial mass. In other words, when the tilt angle θof the inertial mass increases, also the sheer stress σ in thecantilever beam and the internal mass increases, resulting in therupture of the structure (see FIG. 2).

A plurality of small-size, low-cost acceleration sensors oriented indifferent directions can be constructed according to the MEM process onone and the same silicon wafer. Though the best wafer material is singlecrystal silicon, other breakable materials can also come into question,such as silicon dioxide (SiO₂), silicon nitride (SiN), polycrystallinesilicon, quartz, sapphire, or the construction of such materialsprimarily in a form of sandwich structure. Thus, any suitable brittlematerial can be used.

FIG. 4 a illustrates a typical stress-strain curve for polycrystallinematerials and 4 b for a single crystal material such as single crystalsilicon. FIG. 4 a shows that after the yield point the materialplastically elongates until it ruptures, the stress level remainingrelatively constant. A deformed shape of the material remains whenelongates stress applied to it is released. FIG. 4 b shows that when theelongated stress is released the single crystal material returns to itsoriginal shape, i.e. no deformation nor hysteresis is found.

The rupture point shows where the material ruptures. Strain ε is definedbyε=ΔL/L,

-   -   where L represents the length of the body and ΔL is the change        in the length.

Advantages which make silicon the best material for the sensormanufactured according to the MEM process are that silicon iseconomical, ruptures without deformation, is simple to produce in smallsize (area about 1 mm²), and suitable for mass production.

Silicon is a nonmetallic element that is abundantly available. It has adiamond crystal lattice. Especially single crystal silicon isrecommended because its rupture points are easily predictable fromcrystal structure. However, this does not restrict the use of amorphoussilicon (not crystalline on any significant scale), which is much lessexpensive material than single crystal silicon.

The process used enables the manufacturing of three dimensionalacceleration sensors using surface acceleration sensors on the samesilicon wafer.

Electrochemical etching is one of the techniques used for depositing andpatterning the surface of the silicon wafer. Two different etchingtechniques are proposed to remove material: wet and dry etching. Thesimplest structures that can be formed on the silicon wafer are V shapedgrooves or holes with right-angled corners and sloping walls.

FIG. 5 illustrates one example of a structure including more than oneinertial mass etched on the same chip. In this example the structuremicromachined on a silicon wafer has the shape of an L-letter and iscomprised of a frame 500 and two cubical inertial masses 501 and 502,each of which have two tiny cantilever beams 503 symmetrically at theopposite corners on the same edge of the inertial mass. At least part ofthe said structure is covered with a conductive layer 504.

Both of the arms of the L-structure have a cavity with right-angledcorners and straight walls. One inertial mass is in each cavity. One endof the cantilever beam is connected to the inertial mass and the otherend to the wall of the cavity. The cantilever beams of the inertial mass501 are in parallel direction with the x-axis and the cantilever beamsof the inertial mass 502 are in the direction parallel with the y-axisas shown in FIG. 5. As stated above, the relative dimensions of thecomponents shown in the figure may vary in reality. For example, thecantilever beams are very tiny in comparison with the inertial mass. Theinterface between the cantilever beam and the frame, as well as theinterface between the cantilever beam and the inertial mass, is unbrokenbecause the said structure is etched on the same chip. Of course,depending on the material used and the size of the structure, thecantilever beam can also be a separate part that is connected in somesuitable way to the inertial mass and the frame.

At least part of the structure is covered with breakable conductivematerial 504. The shape and the location of it is not restrictedproviding that the conductive material is arranged in such a way that itbreaks when the cantilever breaks. According to FIG. 5 an area 505 ofthe frame top surface is reserved for electronic circuitry.

Generally, the sensitivity of the structure is increased proportionallyto the number of inertial masses it has with different accelerationthreshold levels and tilting directions. The invention includes severalways of implementing the presented inertial masses by orienting them indifferent ways.

For example, in some of the ways six inertial masses can be arranged toform a hexagonal structure or three inertial masses can be oriented insuch a way that the second inertial mass is turned 45 degrees in respectto the first inertial mass and the third is turned 45 degrees in respectto the second.

FIG. 6 shows a flow diagram of an exemplary fabrication method for theinertial sensor.

A SOI wafer 600 is used in the manufacturing process described. First, amask layer is deposited on the wafer with spin casting or according toother methods at stage 601. Mask is then developed with photolithographyto form open areas for the conductive path. After mask development theconductive layer is deposited over the wafer at stage 602. The masklayer is then removed from the wafer and the conductive path has beenformed on the wafer, stage 603.

At the next process stage 604 mask layers for the deep etching aredeposited on both sides of the wafer. The mask openings for the deepetching are processed at stage 605. The inertial mass is then formedfrom the handle layer of the wafer with deep etching from the back sideof the wafer at process stage 606. The cantilever beams are formed fromthe device layer of the wafer with etching from the front side of thewafer at stage 607. Before releasing the device, the mask layers areremoved from the back and front sides of the wafer at process stage 608.Finally, at stage 609 the device is released by etching the insulatorfrom open areas where it holds the inertial mass.

FIG. 7 a-b shows a cross section, the front side and the back side ofthe SOI wafer after each process phase. The process flow stages 700-709in FIG. 7 a-b correspond to the process stages 600-609 in FIG. 6 and aredescribed in more detail in the following.

The SOI wafer consists of two silicon layers 70D and 70H (a device layerand a handle layer), and an insulator layer (SiO) 71 at stage 700. Themask layer 72 is deposited on the silicon layer 70D. At stage 701 anopen area 74 for the conductive path is seen in the front side figure.The (noble) metal 73 is then evaporated (or deposited in any othersuitable way) on the mask layer 72 (at stage 702) so that when the masklayer is removed at stage 703 a pattern consisting of the evaporatedmetal 73 remains on the wafer surface 70D. At the next stage 704 a masklayer 72 is deposited on both silicon layers, i.e. on layer 70D andlayer 70H. The first layer 70H is deeply etched (stage 706) to form theinertial mass 75. Then layer 70D is etched to form two cantilever beams76 at stage 707. Removal of material is performed by a dry etchingtechnique such as reactive ion etching, which is the most common form ofdry etching for micromachining applications. Now the device is formedfrom the SOI wafer and is held with the insulator layer 71 between thehandle and device layers, i.e. layers 70H and 70D. The mask layers 72are removed (stage 708), and the insulator layer 71 is etched away fromthe open areas 77 at stage 709.

FIG. 8 shows one example of the breakable acceleration sensor. In FIG. 8a there is a Silicon On Insulator (SOI) as seen from above, consistingof two silicon layers 800 and 809 and a silicon oxide layer 807 betweenthem.

FIG. 8 b illustrates the same SOI after removal of material (i.e. partof layers 807 and 809) by the dry etching technique. The pattern createdis multiform comprising two areas 801 and 802 rising from the siliconsubstrate 800, an inertial mass 804 essentially apart from the saidsilicon substrate, a tiny cantilever beam 805 with one end connected tothe inertial mass and the other end to the area 801, and a bridge 806including a conductive material and connecting the said areas. Thesilicon oxide layer is otherwise removed throughout the SOI but leftunder the areas 801 and 802.

The inertial mass is at a distance from the bridge with no obstaclesbetween them, so that when the sensor is sufficiently accelerated theinertial mass operates as a hammer breaking the bridge 806. The breakingpoint 808 is shown with a dotted circle in FIG. 8 b. Break down of thebridge is easily discovered when electrical conductivity is measuredthrough two terminals 803, one of which is on area 801 and the other onarea 802.

FIG. 8 c shows a cross section of the structure in FIG. 8 b as seen inthe y-direction from the dotted line 810. In this figure it is easilyseen that both the inertial mass 804 and the cantilever beam 805 areapart from the silicon substrate 800.

FIG. 9 shows a typical embedded computer system comprising a processor901, a display 902, a memory 903, a real-time clock 904, a signalingmeans 905 for signaling power-up and/or power-down to the processor, anddata exchanging means 906. At least a part of the memory 903 isnonvolatile, and another part is reserved for instructions that theprocessor executes.

The example system according to FIG. 9, which is arranged to measureacceleration sensor fuse loops, comprises a first loop 911 functionallyconnected through a first acceleration sensor 921 to the processor 901,a registering means 907 for registering signals received from the firstloop, a reading means 908 for reading data 909 from the registeringmeans, and an interrupting means 910 for interrupting the normalexecution of said instructions of a stored program in said memory 903.

The first loop 911 is advantageously buffered by a buffer 920 beforesaid loop is latched in the register 907. This register is typicallyreset when data 909 is read by the reading means 908.

In an alternative embodiment the buffer 920 as well as the registeringmeans 907 can be omitted, whereby the signal from the first loop isdirectly connected to the processor. Such connection is also shown fortwo additional loops, i.e. a second loop 912 and a third loop 913. Thesecond loop is functionally connected through a second accelerationsensor 922 to the processor 901. Similarly the third loop isfunctionally connected to the processor by looping the third loopthrough both a third acceleration sensor 923 and sensor 922. Theacceleration sensors are of the kind described above, for example.

Breakage of the 911 loop is reported by an interrupt request signal 910.Alternatively the loop is interrogated periodically by polling. Pollingis advantageously used at power up and power down.

The status of the various loops, such as loops 911, 912, and 913 in FIG.9, are advantageously determined by polling during a self-test procedureat power up and at power down. The present status is time-stamped andregistered in the memory using time information from the real-time clock904.

Typically only the last two time-stamped power-down and power-up statusevents are registered in the non-volatile memory for each loop, and eachtime-stamped data overwrites the previously registered time-stamped dataif the result of the self-test is the same as previously. Thus, nomemory space is wasted unnecessarily, but any acceleration event causinga loop to break will have its identity time-stamped with either theexact break time or be time framed by two time-stamps. These time-stampsare the power-down event before the break and the power-up event laterwhen a loop break is discovered, for example, by polling during thepower-on self-test procedure that checks the status of the variousloops.

The program running in the processor can inform the user about aacceleration event and its implication using the display 902, but thisis dependent on the application and whether a display is available.

The resistance of the various loops can be measured using an internalAnalog to Digital Converter (ADC) in the processor 901, or some othersystem resource, and the measured value is compared with a previouslyregistered value. If the change exceeds a predetermined minimum value,the result is time stamped and registered. Advantageously only a voltagelevel is used to indicate the status of a loop, and a binary change willcause that the result is time stamped and registered.

In arranging suitable test points, the resistance of the various loopscan also be measured by field maintenance using a suitableVolt-Ohm-Meter (VOM). This is advantageous if the processor has ceasedfunction.

In some cases, especially when multiple acceleration sensors are used,it might be advantageous to use two parallel breakable paths, hereaftercalled fuses, through an acceleration sensor, as shown with accelerationsensor 922 in FIG. 9.

Generally the use of parallel fuses minimizes interaction betweenvarious readout methods. For example, one serial loop can be connectedto a digital input port of the processor and other breakable fuses canbe connected in parallel before connected to an analogue input port. Ifeach of these parallel loops has a resistor in a series with a fuse, thevalue of the resistor having been selected from a binary sequence, it ispossible to determine exactly which of 32 sensors, for example, arebroken and which are not. If each of the paralleled fuses has a resistorserially inserted, selected in binary sequence by conductance, it ispossible to determine from the resulting resistance which of the fusesare broken and which fuses remain intact. The resistance value can evenbe read with a simple multimeter if suitable contact pads have beenarranged to facilitate measuring. Even with standard 2% resistors, it ispossible to determine, for example, exactly which of 32 sensors arebroken and which are not.

It is advantageous to have at least one loop that can be read even ifthe processor 901 is non-operative, due to lack of power, for example.Parallel breakable fuses can thus advantageously be used when it isdesired that some of the loops are to be remotely readable. Thistechnique is described later in more detail.

Instead of having multiple breakable fuses, additional simplecombinatorial logic in the registering means 907 can be used todetermine if a broken fuse belongs to a set of fuses that causes awarning or to a set of fuses that will indicate that the warranty hasbeen forfeited.

Because the invented sensors are sensitive in multiple planes,insensitive only in the direction of the fuse shaft, very few sensorsare needed to give multidirectional shock coverage. In FIG. 9 thesensors 922 and 923 are shown in a 90-degree relative position. Thisgives a good multi-angle coverage, but using three sensors in a delta orY configuration, is optimal for practical directional insensitivity.

Generally the deflection of the inertial mass in an acceleration sensordepends both on the acceleration and the mechanical self-resonance ofthe inertial mass structure. Even small amplitude vibrations occurringat the self-resonance frequency of the system will cause largedeflections that will break the cantilever beam. This must be taken intoaccount when the system is designed.

The amplitude of environmental vibrations with a frequency over a fewkilohertz, occurring for example in various vehicles, is howeverinsignificant compared to the shock accelerations the inventedaccelerometers are designed to detect. The self-resonance frequency forthe invented sensors is in the high kilohertz range, typically 6 kHz.

FIG. 10 exemplifies remote sensing using RF resonance circuits. In FIG.10 each of the breakable fuses 951, 952, and 953 are double fuses. Oneof the double fuses is used to form a serially connected self-test loop50. This loop is functionally connected to the processor 901 and can beread by a processor 901 as previously described.

The remaining three fuses of the double fuses are each separatelyserially connected with a capacitor 961, 962 and 963 as well asconnected in parallel amongst themselves and with a capacitor 960 and aninductive loop 981. If no fuse is broken, all four capacitors 960, 961,962, and 963 are paralleled and with the inductance of the inductiveloop 981 form a resonance circuit. If any of the capacitors 961, 962,and 963 are switched out of the resonance circuit, typically when acorresponding fuse is broken, the resonance frequency of the resonancecircuit, now formed by the inductive loop 981 and the capacitor 960paralleled with the remaining sensor capacitors, will increase. If thecapacitors 961, 962, and 963 are selected from a suitable sequence, theresonance frequency of each combination of broken and unbroken fuseswill be different.

It is advantageous to include the capacitor 960 because it willguarantee a measurable output frequency even if all fusible links arebroken. Otherwise, in the case when all fuses are broken, doubt wouldremain whether the measurement method itself is working.

The said resonance frequency can be remotely read if the resonancecircuit is activated from outside. This is the preferred method when theequipment is brought in for warranty repairs, because the status of thebreakable fuses, or at least those directly affecting warranty, can bedetermined even without opening the equipment. If the equipment isopened, an alternative method is to measure the resistance of the loop950 using an extemal VOM (Volt Ohm Meter) 975 connected to the ports 971and 972, arranged to be accessible. The drawback is that exactly whichthe fuse in the loop was broken cannot be determined, but in most casesthis is not necessary.

When activating the said resonance circuit from the outside, asubstantially similar inductive loop 983 is brought into range 982 ofthe internal inductive loop 981. The inductive loop 983 is functionallyconnected either to a swept oscillator 993, a noise source 993, or apulse generator 993. The simplest way to measure the resonance frequencyof the resonance circuit that includes the inductance loop 981 is tosweep through the possible frequency range and observe the AC voltmeter991 for resonance peaks occurring at a certain frequency, as indicatedby the frequency analyzer 992, or by any frequency meter. If thefrequencies corresponding to all combinations of broken fuses are listedbeforehand by the equipment manufacturer, it is a simple matter forfield maintenance to determine which fuse or fuses are broken.

A more advanced method is to use a noise or pulse generator instead ofthe sweeping generator 993. The resonance frequencies can then directlybe observed on a frequency analyzer 992, and the broken fuses can bedetermined using the same list as before. If the ports 973 and 974 areaccessible, the inductive loop 983 is not even necessary; because theports 994 and 995 of the activating generator 993 can be directlyconnected to the ports 973 and 974, of the resonance circuit.

FIG. 11 a-b is an example of an acceleration sensor processed using bothbulk-micro-mechanic and surface micro-mechanic techniques. Theprocessing can be carried out using a Silicon On Insulator wafer (as inFIG. 11 a-b), but also conventional silicon wafer can come intoquestion.

FIG. 11 a is a cross-section view of an acceleration sensor processed ona Silicon On Insulator wafer. A cubical silicon inertial mass 1100 isinterconnected to a frame 1101 by the polysilicon conductor making theinertial mass integral with the frame. Said conductor covers also a partof the surface of the inertial mass and a part of the said frame. Aninsulator layer 1103 isolates the polysilicon conductor from the siliconframe. Two metal pads are processed on the polysilicon conductor. Theacceleration sensor is connected to a reading electronic part by thesepads.

FIG. 11 b is the same acceleration sensor as seen from above, i.e. fromthe direction which is perpendicular to the surface of polysiliconconductor.

When acceleration applied to the acceleration sensor exceeds apredetermined threshold level, the polysilicon conductor breaks atpoints 1102, shown with dotted circles in FIG. 11 a and 11 b. The padsare located on the polysilicon conductor in such a way that the breakingpoints are between them, so that electrical conductivity ornon-conductivity between the pads indicates that the interconnectionbetween the inertial mass and the frame is unbroken or broken.

FIG. 12 shows in a side view a sensor system. Such sensor system can bea stand-alone module, a multichip module (MCM) or a integrated circuitin the form of a System on a Chip (SoC).

FIG. 12 a exemplifies the sensor system 611 on a substrate 620. Theinertial mass is shown as 612. The insulator layer 613 isolates themetal layer 615 that forms the breakable fuse on top of the cantileverbeam 614. The fuse is connected through the pads 616, bonding wire 617and pads 618 to the substrate and using conductive tracks from the pads618 to the pads 621 of the measuring and registering circuitry 611. Thiscircuitry can either be a standalone system or be part of a host system.Any needed serial or parallel communications bus can be formed usingadditional pads. The pads 623 on the substrate allow direct measuringusing a VOM meter.

FIG. 12 b exemplifies when the measuring circuitry 622 is contained inthe sensor itself. The measuring circuitry is now contained in the area622 of FIG. 12 b. This is the corresponding area to the previouslydiscussed circuitry area 505 in FIG. 5. The pads 618 can be used toarrange a suitable parallel or serial communications bus, and loops canbe brought out through additional pads to measurements pads 613. Theinductive loop 981 can in the same way advantageously be arranged astracks on the substrate 620 if the dimensions are such that it isimpractical to arrange it in the sensor system 611.

Several sensors having different inertial threshold value can be placedin a product or device. For example, one of the sensors may have lowerthreshold value (e.g. 100 g) than the others so that it givesforewarning to the user to handle the device more carefully. However,even when the device has already broken, it can indicate to the user orto the repairman when and in what way breakage came about. Informationabout breakage such as time and amount of acceleration is stored in anon-volatile register. If the breakage is so severe that it prevents anynormal usage of the device, the status of the fuses can be readpassively instead. After repair, it might be possible to recover moreinformation about the breakage.

Although the invention was described above with reference to theexamples shown in the appended drawings, it is obvious to theprofessional that the invention can be changed within the scope of theinventive idea presented above and in the appended claims. For example,the sensor and the reading method of the sensor fuses are suitable to beused in all kinds of products, especially electrical equipment such asmobile phones, microphones, and electronic books. The accelerationsensor can be used to inform a customer that a product has suffered anacceleration shock and ask that the product be checked in a repair shop.The sensor can also warn the user that the product has been subjected toan acceleration shock close to the warranty limit. More positively, itcan testify that the equipment did not suffer any acceleration shockwhen in temporary use by somebody else.

Other applications are found in the logistic chain: tracing themishandling of packages and containers, courier services, etc., as wellas guaranteeing that the customer is receiving a faultless device.

Because the invented acceleration sensor can be built on any substratesuitable for MEMS, they can be integrated with other electronics andneed not even be separate devices.

The inherent repeatability and possibility of measuring acceleration inany direction makes the invented acceleration sensors suitable forcustomization. They can be built to be sensitive in the directionsrelevant to any piece of equipment.

Typically the invented sensors can be used in flip chip or Ball GridArray (BGA) packages, and in other LGA (Land Grid Array) packages thatthrough surface mounting guarantee good mechanical coupling to theobject to be observed. But when the invented sensors are part of alarger circuit, the sensors use the existing interconnection technique.

The above-mentioned conductive material can alternatively beunbreakable, but built into the structure in such a way that no electriccurrent can be observed when the sensor is broken. For example, thedetecting means comprises conductive path, strip, wire, doped-silicon,or polycrystalline silicon at least on the interconnecting element.

1. An acceleration sensor arrangement, an acceleration sensor comprisinga first body portion, a second body portion, an interconnecting elementmaking the first body portion integral with the second body portion anddetecting means arranged for giving an indication when the second bodyportion damages the detecting means, characterized in that theacceleration sensor arrangement comprises a group of at least two saidacceleration sensors arranged on one carrier, at least two of thesensors responding to different forces:
 2. The acceleration sensorarrangement as in claim 1, wherein the group comprises accelerationsensors responding to forces in at least three different directions. 3.The acceleration sensor arrangement as in claim 1, wherein the detectingmeans comprises a conductive path, strip, or wire arranged at least onthe interconnecting element.
 4. The acceleration sensor arrangement asin claim 1, wherein the detecting means comprises a conductivedoped-silicon or polycrystalline silicon layer at least on theinterconnecting element.
 5. The acceleration sensor arrangement as inclaim 3, wherein the interconnecting element is adapted to break when anexternal force affecting the second body portion of the accelerationsensor exceeds a predetermined threshold level, wherein a break of theinterconnecting element causes a break in the conductive path, strip,wire, or layer.
 6. The acceleration sensor arrangement as in claim 1,wherein the detecting means comprises a conductive strip or wirearranged at a distance from the second body portion, wherein the secondbody portion of the acceleration sensor moves and breaks the path,strip, or wire when an external force affecting the second body portionexceeds a predetermined threshold level.
 7. The acceleration sensorsarrangement as in claim 1, wherein the detecting means from a part of anelectrical detection loop.
 8. The acceleration sensor arrangement as inclaim 1, wherein the indication is stored in a memory.
 9. Theacceleration sensor arrangement as in claim 1, wherein the indication isremotely readable.
 10. The acceleration sensor arrangement as in claim1, wherein the acceleration sensor is produced by micromachiningtechnology using a surface mountable brittle material.
 11. Theacceleration sensor arrangement as in claim 10, wherein the brittlematerial is single crystal silicon.
 12. The acceleration sensorarrangement as in claim 10, wherein the brittle material ispolycrystalline silicon.
 13. The acceleration sensor arrangement as inclaim 1, wherein the indication contains at least informationidentifying a detecting loop broken by an external acceleration force.14. The acceleration sensor arrangement as in claim 13, wherein theindication further contains the time when the indication was given. 15.The acceleration sensor arrangement as in claim 1, wherein the status ofthe acceleration sensor arrangement is readable immediately or from thememory.
 16. The acceleration sensor as in claim 15, wherein at least oneof the acceleration sensors in the arrangement is adapted to give awarning to the user when an external force affecting the second bodyportion exceeds a predetermined threshold level.
 17. The accelerationsensor arrangement as in claim 1, wherein all sensors of the arrangementare integrated in a single block.
 18. The acceleration sensorarrangement as in claim 1, wherein the acceleration of any of thesensors of the arrangement is remotely identifiable.
 19. Theacceleration sensor arrangement as in claim 17, wherein the single blockfurther comprises means for storing indications containing at least thetime when the indication was given and the identity of the detectingmeans.
 20. The acceleration sensor arrangement as in claim 1, whereinall sensors of the arrangement are integrated in a multichip moduletogether with means for storing indications containing at least the timewhen the indication was given and the identity of the detecting means.21. The acceleration sensor arrangement as in claim 1, wherein allsensors of the arrangement are integrated in an integrated circuittogether with means for storing indications containing at least the timewhen the indication was given and the identity of the detecting means.22. A handheld terminal, characterized by an acceleration sensorarrangement comprising a group of at least two acceleration sensors, atleast two of the sensors responding to different forces, an accelerationsensor comprising a first body portion, a second body portion, aninterconnecting element making the first body integral with the secondbody, and detecting means arranged for giving an indication when thesecond portion damages the detecting means and further giving anindication to the terminal user of the event.
 23. The handheld terminalas in claim 22, wherein the acceleration sensor arrangement beingarranged to indicate to the terminal when an acceleration forceaffecting at least one acceleration sensor of the arrangement exceeds apredetermined threshold level and to give a warning to a user of theterminal if said indicating is active when the terminal is witched on.24. A method in an acceleration sensor arrangement comprising a group ofat least two acceleration sensors, an acceleration sensor comprising afirst body portion, at least one second body portion and aninterconnecting element making the first body portion integral with theat least one second body portion, the method comprising giving anindication when a second body portion of at least one accelerationsensor of the arrangement damages at least one detecting means.
 25. Themethod as in claim 24, the method further comprising registering in anon-volatile memory last two time-stamped power-down and power-up statusevents by overwriting a previously registered time-stamped data as longas the status of the detecting means of the acceleration sensorarrangement is the same as previously.